The input parameters are declared the same way as variables are. As an
exception, unused parameters can omit the variable name.
For example, suppose we want our contract to
accept one kind of external calls with two integers, we would write
something like:

pragmasolidity^0.4.16;contractSimple{functiontaker(uint_a,uint_b)publicpure{// do something with _a and _b.}}

The output parameters can be declared with the same syntax after the
returns keyword. For example, suppose we wished to return two results:
the sum and the product of the two given integers, then we would
write:

The names of output parameters can be omitted.
The output values can also be specified using return statements.
The return statements are also capable of returning multiple
values, see Returning Multiple Values.
Return parameters are initialized to zero; if they are not explicitly
set, they stay to be zero.

Input parameters and output parameters can be used as expressions in
the function body. There, they are also usable in the left-hand side
of assignment.

Most of the control structures from JavaScript are available in Solidity
except for switch and goto. So
there is: if, else, while, do, for, break, continue, return, ?:, with
the usual semantics known from C or JavaScript.

Parentheses can not be omitted for conditionals, but curly brances can be omitted
around single-statement bodies.

Note that there is no type conversion from non-boolean to boolean types as
there is in C and JavaScript, so if(1){...} is not valid
Solidity.

These function calls are translated into simple jumps inside the EVM. This has
the effect that the current memory is not cleared, i.e. passing memory references
to internally-called functions is very efficient. Only functions of the same
contract can be called internally.

The expressions this.g(8); and c.g(2); (where c is a contract
instance) are also valid function calls, but this time, the function
will be called “externally”, via a message call and not directly via jumps.
Please note that function calls on this cannot be used in the constructor, as the
actual contract has not been created yet.

Functions of other contracts have to be called externally. For an external call,
all function arguments have to be copied to memory.

When calling functions of other contracts, the amount of Wei sent with the call and
the gas can be specified with special options .value() and .gas(), respectively:

The modifier payable has to be used for info, because otherwise, the .value()
option would not be available.

Note that the expression InfoFeed(addr) performs an explicit type conversion stating
that “we know that the type of the contract at the given address is InfoFeed” and
this does not execute a constructor. Explicit type conversions have to be
handled with extreme caution. Never call a function on a contract where you
are not sure about its type.

We could also have used functionsetFeed(InfoFeed_feed){feed=_feed;} directly.
Be careful about the fact that feed.info.value(10).gas(800)
only (locally) sets the value and amount of gas sent with the function call and only the
parentheses at the end perform the actual call.

Function calls cause exceptions if the called contract does not exist (in the
sense that the account does not contain code) or if the called contract itself
throws an exception or goes out of gas.

Warning

Any interaction with another contract imposes a potential danger, especially
if the source code of the contract is not known in advance. The current
contract hands over control to the called contract and that may potentially
do just about anything. Even if the called contract inherits from a known parent contract,
the inheriting contract is only required to have a correct interface. The
implementation of the contract, however, can be completely arbitrary and thus,
pose a danger. In addition, be prepared in case it calls into other contracts of
your system or even back into the calling contract before the first
call returns. This means
that the called contract can change state variables of the calling contract
via its functions. Write your functions in a way that, for example, calls to
external functions happen after any changes to state variables in your contract
so your contract is not vulnerable to a reentrancy exploit.

Function call arguments can also be given by name, in any order,
if they are enclosed in {} as can be seen in the following
example. The argument list has to coincide by name with the list of
parameters from the function declaration, but can be in arbitrary order.

A contract can create a new contract using the new keyword. The full
code of the contract being created has to be known in advance, so recursive
creation-dependencies are not possible.

pragmasolidity^0.4.0;contractD{uintx;functionD(uinta)publicpayable{x=a;}}contractC{Dd=newD(4);// will be executed as part of C's constructorfunctioncreateD(uintarg)public{DnewD=newD(arg);}functioncreateAndEndowD(uintarg,uintamount)publicpayable{// Send ether along with the creationDnewD=(newD).value(amount)(arg);}}

As seen in the example, it is possible to forward Ether while creating
an instance of D using the .value() option, but it is not possible
to limit the amount of gas.
If the creation fails (due to out-of-stack, not enough balance or other problems),
an exception is thrown.

The evaluation order of expressions is not specified (more formally, the order
in which the children of one node in the expression tree are evaluated is not
specified, but they are of course evaluated before the node itself). It is only
guaranteed that statements are executed in order and short-circuiting for
boolean expressions is done. See Order of Precedence of Operators for more information.

Solidity internally allows tuple types, i.e. a list of objects of potentially different types whose size is a constant at compile-time. Those tuples can be used to return multiple values at the same time and also assign them to multiple variables (or LValues in general) at the same time:

pragmasolidity^0.4.16;contractC{uint[]data;functionf()publicpurereturns(uint,bool,uint){return(7,true,2);}functiong()public{// Declares and assigns the variables. Specifying the type explicitly is not possible.var(x,b,y)=f();// Assigns to a pre-existing variable.(x,y)=(2,7);// Common trick to swap values -- does not work for non-value storage types.(x,y)=(y,x);// Components can be left out (also for variable declarations).// If the tuple ends in an empty component,// the rest of the values are discarded.(data.length,)=f();// Sets the length to 7// The same can be done on the left side.// If the tuple begins in an empty component, the beginning values are discarded.(,data[3])=f();// Sets data[3] to 2// Components can only be left out at the left-hand-side of assignments, with// one exception:(x,)=(1,);// (1,) is the only way to specify a 1-component tuple, because (1) is// equivalent to 1.}}

The semantics of assignment are a bit more complicated for non-value types like arrays and structs.
Assigning to a state variable always creates an independent copy. On the other hand, assigning to a local variable creates an independent copy only for elementary types, i.e. static types that fit into 32 bytes. If structs or arrays (including bytes and string) are assigned from a state variable to a local variable, the local variable holds a reference to the original state variable. A second assignment to the local variable does not modify the state but only changes the reference. Assignments to members (or elements) of the local variable do change the state.

A variable which is declared will have an initial default value whose byte-representation is all zeros.
The “default values” of variables are the typical “zero-state” of whatever the type is. For example, the default value for a bool
is false. The default value for the uint or int types is 0. For statically-sized arrays and bytes1 to bytes32, each individual
element will be initialized to the default value corresponding to its type. Finally, for dynamically-sized arrays, bytes
and string, the default value is an empty array or string.

A variable declared anywhere within a function will be in scope for the entire function, regardless of where it is declared
(this will change soon, see below).
This happens because Solidity inherits its scoping rules from JavaScript.
This is in contrast to many languages where variables are only scoped where they are declared until the end of the semantic block.
As a result, the following code is illegal and cause the compiler to throw an error, Identifieralreadydeclared:

// This will not compilepragmasolidity^0.4.16;contractScopingErrors{functionscoping()public{uinti=0;while(i++<1){uintsame1=0;}while(i++<2){uintsame1=0;// Illegal, second declaration of same1}}functionminimalScoping()public{{uintsame2=0;}{uintsame2=0;// Illegal, second declaration of same2}}functionforLoopScoping()public{for(uintsame3=0;same3<1;same3++){}for(uintsame3=0;same3<1;same3++){// Illegal, second declaration of same3}}}

In addition to this, if a variable is declared, it will be initialized at the beginning of the function to its default value.
As a result, the following code is legal, despite being poorly written:

Starting from version 0.5.0, Solidity will change to the more widespread scoping rules of C99
(and many other languages): Variables are visible from the point right after their declaration
until the end of a {}-block. As an exception to this rule, variables declared in the
initialization part of a for-loop are only visible until the end of the for-loop.

Variables and other items declared outside of a code block, for example functions, contracts,
user-defined types, etc., do not change their scoping behaviour. This means you can
use state variables before they are declared and call functions recursively.

These rules are already introduced now as an experimental feature.

As a consequence, the following examples will compile without warnings, since
the two variables have the same name but disjoint scopes. In non-0.5.0-mode,
they have the same scope (the function minimalScoping) and thus it does
not compile there.

As a special example of the C99 scoping rules, note that in the following,
the first assignment to x will actually assign the outer and not the inner variable.
In any case, you will get a warning about the outer variable being shadowed.

pragmasolidity^0.4.0;pragmaexperimental"v0.5.0";contractC{functionf()purepublicreturns(uint){uintx=1;{x=2;// this will assign to the outer variableuintx;}returnx;// x has value 2}}

Solidity uses state-reverting exceptions to handle errors. Such an exception will undo all changes made to the
state in the current call (and all its sub-calls) and also flag an error to the caller.
The convenience functions assert and require can be used to check for conditions and throw an exception
if the condition is not met. The assert function should only be used to test for internal errors, and to check invariants.
The require function should be used to ensure valid conditions, such as inputs, or contract state variables are met, or to validate return values from calls to external contracts.
If used properly, analysis tools can evaluate your contract to identify the conditions and function calls which will reach a failing assert. Properly functioning code should never reach a failing assert statement; if this happens there is a bug in your contract which you should fix.

There are two other ways to trigger exceptions: The revert function can be used to flag an error and
revert the current call. In the future it might be possible to also include details about the error
in a call to revert. The throw keyword can also be used as an alternative to revert().

Note

From version 0.4.13 the throw keyword is deprecated and will be phased out in the future.

When exceptions happen in a sub-call, they “bubble up” (i.e. exceptions are rethrown) automatically. Exceptions to this rule are send
and the low-level functions call, delegatecall and callcode – those return false in case
of an exception instead of “bubbling up”.

Warning

The low-level call, delegatecall and callcode will return success if the called account is non-existent, as part of the design of EVM. Existence must be checked prior to calling if desired.

Catching exceptions is not yet possible.

In the following example, you can see how require can be used to easily check conditions on inputs
and how assert can be used for internal error checking:

pragmasolidity^0.4.0;contractSharer{functionsendHalf(addressaddr)publicpayablereturns(uintbalance){require(msg.value%2==0);// Only allow even numbersuintbalanceBeforeTransfer=this.balance;addr.transfer(msg.value/2);// Since transfer throws an exception on failure and// cannot call back here, there should be no way for us to// still have half of the money.assert(this.balance==balanceBeforeTransfer-msg.value/2);returnthis.balance;}}

An assert-style exception is generated in the following situations:

If you access an array at a too large or negative index (i.e. x[i] where i>=x.length or i<0).

If you access a fixed-length bytesN at a too large or negative index.

If you divide or modulo by zero (e.g. 5/0 or 23%0).

If you shift by a negative amount.

If you convert a value too big or negative into an enum type.

If you call a zero-initialized variable of internal function type.

If you call assert with an argument that evaluates to false.

A require-style exception is generated in the following situations:

Calling throw.

Calling require with an argument that evaluates to false.

If you call a function via a message call but it does not finish properly (i.e. it runs out of gas, has no matching function, or throws an exception itself), except when a low level operation call, send, delegatecall or callcode is used. The low level operations never throw exceptions but indicate failures by returning false.

If you create a contract using the new keyword but the contract creation does not finish properly (see above for the definition of “not finish properly”).

If you perform an external function call targeting a contract that contains no code.

If your contract receives Ether via a public function without payable modifier (including the constructor and the fallback function).

If your contract receives Ether via a public getter function.

If a .transfer() fails.

Internally, Solidity performs a revert operation (instruction 0xfd) for a require-style exception and executes an invalid operation
(instruction 0xfe) to throw an assert-style exception. In both cases, this causes
the EVM to revert all changes made to the state. The reason for reverting is that there is no safe way to continue execution, because an expected effect
did not occur. Because we want to retain the atomicity of transactions, the safest thing to do is to revert all changes and make the whole transaction
(or at least call) without effect. Note that assert-style exceptions consume all gas available to the call, while
require-style exceptions will not consume any gas starting from the Metropolis release.